Continuous cellulose digester with two separate pulp outlets

ABSTRACT

A CONTINUOUS CELLULOSE DIGESTER IS PROVIDED WITH TWO PULP DISCHARGING MEANS, ONE DISCHARGING THE CORE OF THE MOVING PLUG OF CELLULOSIC MATERIAL THROUGH A FIRST OUTLET AND THE OTHER DISCHARGING THE PERIPHERAL PARTS OF THE PLUG THROUGH A SECOND OUTLET. THE FIRST DISCHARGING MEANS MAY BE A ROTARY SCRAPER WORKING IN THE CENTER AREA ONLY OF THE OUTLET END OF THE DIGESTER, THE SECOND DISCHARGING MEANS WORKING IN THE SURRONDING ANNULAR SPACE. THE FIRST OUTLET AND DISCHARGING MEANS IS SEPARATED FROM THE SECOND OUTLET AND DISCHARGING MEANS BY A PARTITION MEANS WHICH IS CONCENTRIC TO THE DIGESTER WALL.

y 18, 1971 J. c. F. c. RICHTER 3,579,421

CONTINUOUS CELLULOSE DIGESTER WITH TWO SEPARATE PULP OUTLETS 2 Sheets-Sheet 1 Filed May 13, 1968 IN: rm

MW. mv M 6 a W A H w 2 Sheets-Sheet 2 .Lmumrm v flrruuns J. C. F. C. RICHTER May 18, 1971 CONTINUOUS CELLULOSE DIGESTER WITH TWO SEPARATE PULP OUTLETS Filed May 13, 1968 15mm (ll-TC. Pier/75k er ad ,Alh

United States Patent C T 3,579,421 CONTINUOUS CELLULOSE DIGESTER WITH TWO SEPARATE PULP OUTLETS Johan C. F. C. Richter, St. Jean Cap Ferrat, France, assignor to Aktiebolaget Kamyr, Karlstad, Sweden Filed May 13, 1968, Ser. No. 728,396 Claims priority, application Sweden, May 16, 1967, 6,788/67 Int. Cl. D21c 7/08 US. Cl. 162-237 8 Claims ABSTRACT OF THE DISCLOSURE A continuous cellulose digester is provided with two pulp discharging means, one discharging the core of the moving plug of cellulosic material through a first outlet and the other discharging the peripheral parts of the plug through a second outlet. The first discharging means may be a rotary s raper working in the center area only of the outlet end of the digester, the second discharging means working in the surrounding annular space. The first outlet and discharging means is separated from the second outlet and discharging means by a partition means which is concentric to the digester wall.

The invention relates to an upright cylindrical cellulose digester with devices for bringing a fiber material such as wood chips, mixed with digesting liquor to move axially through the digester with a velocity which is even and essentially uniform over the entire digester cross-section.

In such digesters, the digesting pulp is usually discharged throu-gh a centrally located outlet at one end of the digester by means of a rotary scraping device acting essentially over the entire cross-section of the digester for moving the end layer of the pulp charge towards the outlet in such a manner that the rest of the charge moves as a coherent column or plug, i.e. with the same axial speed over the entire cross-section. Thus in the outlet, pulp from the core zone of the digester will be mixed with pulp emanating from the peripheral zone of the digester, so that a unitary final product is obtained.

However, the chemical reactions do not take place quite uniformly over the cross-section of the digester, i.e. on account thereof that the temperature necessarily is lower at the peripheral parts that at the central parts of the digester cross-section, and also on account thereof that the digesting and washing liquids which are driven through the fiber material column usually are supplied at the center of the digester cross-section and are spread radially outwards. By various measures said inescapable differences may be deliberately increased, so that the core pulp and the sheath pulp obtain clearly different qualities.

In principle the present invention has for its object to separate said two kinds of pulps so that they can be used for different purposes or possibly be made the object of different continued treatments. In accordance therewith, a basic feature of the invention consists in that the digester is equipepd with two separate outlets for digested pulp, one outlet communicating with a central zone of the digester cross-section, whereas the other otulet communicates with a peripheral zone of the digester cross-section, so that pulp that has passed at a smaller distance from the digester shell can be discharged separately from pulp that has passed at a greater distance from said digester shell. Preferably there .is provided a central pulp discharging device, the action of which is limited to the space close to the axis of the digester and which discharges pulp through one only of the two outlets. When said pulp discharging device consists of a rotary scraper attached to a driving shaft coaxial to the digester for discharging pulp out through a bottom outlet, said scraper is given a diam- 3,579,421 Patented May 18, 1971 eter considerably less than the diameter of the cylindrical digester shell, and the space lying between the lower side of the scraper and said bottom outlet is screened ofi from communication with the space in the end portion of the digester lying radially outside of and circumferentially around said first-mentioned space. From said circumferential space, pulp is discharged through the second pulp outlet preferably by means of a peripheral discharging device which also may be designed as a rotary scraper.

The invention will be more closely described herein below with reference to accompanying drawings, in which FIG. 1 diagrammatically shows a continuous cellulose digester to which the invention is applied, and FIG. 2 shows a modification thereof.

The digester shown is essentially of the design described in the US. Pat. No. 3,298,899 and adapted for the performance of a fiber liberating digestion of lignocellulosic fiber material after which there follows liquor removal and a first washing step of the fiber material taking place already in the digester. Thus the digester consists of an upright, at least substantially cylindrical (or somewhat conical) container 11, in which the fiber material moves continuously in the downward direction as a coherent column or plug while various treatments are performed thereupon. The treated material which may be wood chips, shredded straw or Other finely comminuted vegetabilic fiber material, is supplied through a ocnduit 13 and a charging valve 15 at such a rate, that the digester is held almost filled therewith. To the upper end of the digester high pressure steam is supplied through the conduit 17 in order to heat the fiber material and possibly purge the same of air. Gas collected in the digester top is let out through a conduit 19 having a control valve 21 by means of which the pressure of the digester is controlled so to maintain a value of approximately 8 kg./cm.

Digester liquor, such as sulphate liquor, which is supplied by a conduit 23, is pumped by means of a pump 25 through a heat exchanger 27 wherein the liquor is heated to digesting temperature, and then into the digester through a conduit 29 opening out centrally therein. The liquor is spread radially in all directions and pushes before itself such earlier supplied liquor as has already been somewhat cooled by the fiber material, towards a sieve girdle 31 inserted in the digester shell. Said sieve girdle is connected to the inlet side of the pump 25, the displaced liquor being recirculated after heating. By controlling the heating effect taken place in the heat exchanger 27 relatively to the quantity of circulating liquor it is possible to realize that in a central core zone A of the digester a temperature of approximately 170 C., is maintained, whereas the temperature of a surrounding peripheral zone B be comes approximately C. The border between these zones is indicated by dotted lines 33'. However, it is obvious that no sharp temperature border will show in this case but there will be a diffuse and gradual transition.

The charging speed of the fiber material is chosen such that the digesting reaction is finished when the material reaches the level of the sieve girdle 35 inserted in the digester shell. Spent liquor separated from the pulp by said sieve girdle is discharged via a control valve 37 into a flash tank 39, wherefrom the liquor preferably is sent to a plant for recovery of the chemicals thereof. By means of the pump 41 and a tube 43 opening out centrally in the digester the spent liquor is held in circulation in order to equalize the concentration at the level of the sieve girdle 35.

In the lower part of the digester a first washing step of the digested pulp is performed by means of washing liquid, e.g. water of filtrate, taken from thec onduit 45 and pumped by means of the pump 47 through the tube 48 opening out centrally in the digester. A sieve girdle 49 inserted in the digester shell near the lower end thereof is connected to the inlet side of the pump 47, so that a circulation is maintained whereby the wash liquid is spread radially and distributed over the entire digester cross-section at the level of the sieve girdle 49. From there the wash liquid moves countercurrently to the fiber material towards the sieve girdle 35 while replacing and displacing the digesting liquor. Part of the wash liquid accompanies the spent liquor out through the sieve girdle 35. The wash liqiud supplied also serves to cool the pulp in the bottom end of the digester.

In order to practise the invention and be able to separate the core pulp digested at the higher temperature from the sheath pulp digested at the lower temperature, the digester is provided at its lower end with two separate outlets each having its own discharging device. The two outlets are provided upon a particular bottom piece 51 fastened by means of a flange joint 53 to a neck 55 forming the lower end of the digester. One outlet is formed by a circular groove 57 in the upper face of said bottom piece, and a passage 59 extending from said groove first downwardly and then radially outwards. Connected to said passage 59 is a first pulp conduit 61 wherein a throttle valve 63 is inserted for the control of the discharging speed. The second outlet is formed by a similar circular groove 65 shaped in the upper face of the bottom piece and concentric to the groove 57 but having a greater diameter than the latter, and a connected passage 67 ex tending from the bottom of the groove first downwardly and then radially outwards in a direction opposite to the direction of the passage 59 and connected to a second pulp conduit 69 having a throttle valve '71.

The bottom piece 51 has a vertical bore coaxial to the digester, and through said bore there pass a sleeveshaped driving shaft 73 and a solid shaft 75 inserted in the former and coaxial thereto. The shafts 73, 75 are sealed to the bottom piece and to each other by means of stufiiing boxes 77, 79 and are driven each by a separate motor 81, 83 via a gear. Attached to the upper end of the shaft 75 is a scraper 85 having a hub 87 and essentially radially therefrom extending arms 89 which carry obliquely set scraper blades 91. The diameter of the scraper 85 is considerably less than the diameter of the digester shell, e.g. 50 to 80 percent thereof, and corresponds to the diameter of the core a of the chips column which should be discharged separately. The sleeve-like shaft 73 carries a second scraper 93 having a greater diameter in order to be able to act upon the pulp in the peripheral zone B. From the hub of said scraper there extend radial arms 97 which extend beyond the periphery of the scraper 85 and reach forth to the vicinity of the cylindrical shell of the digester. Said arms carry obliquely set scraper blades 95, 99 which move along the digester bottom and forward pulp down into the groove 65, from where the pulp is scraped down into the outlet passage 67 by means of a scraer blade 101. The hub of the lower scraper also carries a screen sleeve 103 concentric to the digester shell and serving to prevent communication between the core zone and the peripheral zone in the digester bottom end and particularly in its tapering neck portion. Said screen sleeve comprises a lower cylindrical portion which moves close to an annular flange 105 shaped on the bottom piece 51 between the grooves 57 and 65. At its upper end the screen sleeve widens like a funnel to a maximum diameter approximately equal to the diameter of the upper scraper 85, where the screen sleeve is integrally united with the scraper arms 97 which admit passage of the pulp between themselves. Between the screen sleeve and the hub there are passages 107 through which the pulp caught within the screen sleeve during the continuous descent of the pulp column, passes down into the groove 57 in the bottom piece. The scraper blades 91 of the upper scraper move quite close to the upper side of the screen sleeve and assist in the downward feed of the pulp through said passages 107. From the groove l 57 the pulp is moved into the outlet passage 59 by means of a scraper blade 109 attached to the lower scraper.

The modification shown in FIG. 2 differs from the embodiment above described merely therein that the sieve girdle 31 has been replaced by a cylindrical sieve 115 which forms the inner wall of the double-walled annular hollow body 117 having a smaller diameter than the diameter of the shell 11 of the digester and being concentric to the digester. The digesting liquor which penetrates through the sieve 115 and enters the interior of the hollow body is led by a conduit 119 inserted radially in the digester and extending out through its shell, to the pump 25 and then on in circulation. Thus the liquor circulation does not extend to the peripheral parts of the digester cross-section, which are situated radially outside the sieve 115, and therefore the digestion will take place in these peripheral parts at a considerabl lower temperature than in the core zone. The transition border 33 between the zones A and B will be more sharply defined than in FIG. 1. The diameter of the upper scraper is chosen approximately equal to the diameter of the hollow body 117. Therefore, said scraper discharges the entire pulp quantity digested at the higher temperature through the outlet 59, whereas the peripheral parts of the pulp column that are not caught inside the edge of the screen sleeve 103, will be discharged by the lower scraper 93 through the outlet 67. Obviously, a provision for preventing mixture of the two kinds of pulps is that the fiber material column is of such a consistency that it moves as a solid plug, i.e. without inner movements. In order to bring about the above condition it may also be necessary to mutually adapt in a proper manner the rotary speeds of the two scrapers and the adjustments of the throttle valves 63, 61.

Within the scope of the invention set forth by the following claims, the above described embodiments may be modified in various respects. Thus, the concentric scrapers may be replaced by discharging devices operat ing in some other manner. Particularly this applies to the peripheral discharging device, which e.g. may consist of a plurality of tangentially directed spray tubes by means of which the part of the pulp column descending outside the screen sleeve is flushed out. In this case, the screen sleeve may be stationary and preferably made cylindrical at its upper free end. By suitable modifications of the discharging devices shown the invention may also be applied in those cases where the fiber material feed through the digester is directed upwardly.

What is claimed is:

1. A cellulose digesting apparatus comprising an upright-cylindrical cellulose digester including a longitudinally extending shell, said shell having at one end an inlet to feed finely divided fiber material to be digested into said digester and having first and second outlets at the other end which are separate from each other for withdrawing digested pulp from the shell, means for supplying liquor into said material within said digester and means for withdrawing liquor therefrom, said digester including a central treating zone extending longitudinally of the digester and a peripheral treating zone also extending longitudinally of the digester, said peripheral treating zone surrounding said central zone and being positioned between the central zone and the wall of the digester shell, means for feeding said finely divided fiber material mixed with digesting liquor axially through the digester at a speed which is even and essentially uniform over the cross-section of the digester, partition means at said other end of said shell concentric to the wall thereof and of a reduced diameter relative thereto, so as to separate the peripheral zone from the central zone at said other end, one of said outlets being in communication with the central zone of the digester, and said second outlet being in communication with the peripheral zone of the digester, whereby pulp in the peripheral zone of the digester is discharged from the second outlet and pulp in the central zone of the digester is discharged from the first outlet.

2. A digesting apparatus comprising upright-cylindrical cellulose digester including a longitudinal extending shell, said shell having at one end an inlet to feed finely divided fiber material to be digested into said digester and having first and second outlets at the other end which are separate from each other for withdrawing digested pulp from the shell, means for supplying liquor into said material Within said digester and means for withdrawing liquor therefrom, said digester including a central treating zone extending longitudinally of the digester and a peripheral treating zone also extending longitudinally of the digester, said peripheral treating zone surrounding said central zone and being positioned between the central zone and the wall of the digester shell, means for feeding said finely divided fiber material mixed with digesting liquor axially through the digester at a speed which is even and essentially uniform over the cross-section of the digester, one of said outlets being in communication with the central zone of the digester, and said second outlet being in communication with the peripheral zone of the digester, whereby pulp in the peripheral zone of the digester is discharged from the second outlet and pulp in the central zone of the digester is discharged from the first outlet, said apparatus also including central pulp discharging means in the central zone of the digester for discharging pulp only through said first outlet.

3. A digesting apparatus according to claim 2 wherein said discharging means comprises a rotary scraper attached to a driving shaft coaxial to the digester to discharge pulp through said first outlet, said scraper being of smaller diameter than the diameter of the cylindrical digester shell so as to define the peripheral zone outwardly therefrom to the shell wall, the space located between the scraper and said first outlet and constituting part of said central zone being separated from communication with the portion of the peripheral zone of the digester lying radially outside and circumferentially around said first-mentioned space by said partition means,

4. Digester according to claim 3, wherein the outlets are in the bottom end of the shell and said partition means includes a sleeve member which is concentric to the digester shell and widens upwardly, extends from the first outlet upwardly and towards the periphery of the scraper.

5. Digester according to claim 4 including peripheral discharging means, the action of Which is limited to said peripheral zone to move pulp therein only towards said second outlet.

6. Digester according to claim 5 wherein said peripheral discharging means comprises a rotary scraper which is coaxial to the scraper of the central discharging device and which is attached to a further driving shaft which surrounds the driving shaft of the central scraper.

7. Digester according to claim 6 wherein the sleeve is united with one of the rotary scrapers so as to rotate therewith, and the lower end of the sleeve coincides with stationary annular flange means at the digester bottom, one outlet only opening out inside said flange.

8. Digester according to claim 6 wherein the sleeve surrounds and is connected to the further driving shaft and said sleeve forms means for connecting the active parts of the peripheral scraper to said shaft.

References Cited UNITED STATES PATENTS 3,475,271 10/1969 Laakso 162-251 S. LEON BASHORE, Primary Examiner T, G. SCAVONE, Assistant Examiner US. Cl. X.R. 162-246 

